Low conductance exploding bridge



y 1965 'r. c. PARKER ETAL 3,181,464

LOW CONDUCTANCE EXPLODING BRIDGE Filed June 21. 1961 /'4 /a- T I if? p IG-I AffOl/VI) United States Patent "ice 3,181,464 LOW CONDUCTANCE EXPLODING BRIDGE Theodore C. Parker, Sunnyvale, and William H. Colburn, Jr., Irvington, Califi, assignors to General Precision, Inc., a corporation of Delaware Filed June 21, 1961, Ser. No. 118,634 2 Claims. (0.102-28) This invention relates to the use of low conductance, high dielectric threshold conductors in place of high conductance, low dielectric wires as used heretofore in exploding bridge wire devices.

The fields of application of the present invention may be illustrated by the use of an exploding bridge wire (EBW) in explosive and/or detonating devices.

Devices are being made or an increasing scale, wherein a charge of explosive material such as PETN or RDX is initiated by the kinetic and thermal energy created as a result of passage of an electric current at high current density through a wire of small diameter in a very short period of time, of the order of a microsecond; Under the proper conditions a fine wire will melt, vaporize and explode, and the thermal energy and the kinetic energy or shock wave which result will serve to detonate a relatively insensitive explosive such as PETN or RDX.

This technique is advantageous because it avoids the use of a sensitive initiating explosive such as an azide, fulminate or lead styphnate. A much less sensitive explosive such as PETN or RDX can be used, and it is not initiated until the circuit which supplies electrical energy to the bridgewire is closed. The PETN (or other explosive initiated by the exploding bridgewire) may be the principal explosive, but more often it acts as an initiator for the principal explosive (e.g., TNT), or as the initiator of a train of explosive propellant or igniter materials, e.g., to start a'rocket motor.

Nevertheless the EBW has certain serious disadvantages. Being a metallic wire, hence a good conductor, stray or accidental currents may (and not infrequently do) pass through the wire which have the effect of heating the wire. If the degree of heating is small, no harm may be done, but if the temperature of the wire is raised to its fusion point arcing may result and may initiate the explosive, causing premature explosion. Even if premature initiation does not occur, the wire may fuse and be destoryed, thereby converting the device into a dud.

It is an object of the present invention to improve upon electrothermal initiator systems of the general character described.

It is a particular object of the invention to provide a device capable of producing a pulse of thermal and kinetic energy of the general character produced by an EBW, and which is actuated by an EBW power circuit, yet which has a threshold characteristic such that it will not conduct current in any significant degree unless and until the intended functioning conditions are brought into existence.

Yet another object is to produce an EBW type of device wherein there is interposed between two terminals a quasi-conductor which (1) will conduct and will under.- go a rapid change of state accompanied by the release of a large amount of energy when the desired conditions of voltage, voltage rise and electrical energy arebrought into existence but which (2) will not conduct to any substantial degree and will not fuse or otherwise suffer destruction at substantially lowervoltages.

The above and other objects of the invention will be apparent from the ensuing description and the appended claims.

FIGURE 1 is a diagrammatic circuit drawing showing an EBW power source and a bridge element in accordance with the present invention.

3,181,454 Patented May 4, 1965 FIGURE 2 is a detail of FIGURE 1, showing on a larger scale, the bridge element of FIGURE 1.

FIGURE 3 is a view in longitudinal section of an EBW device or commercial igniter which embodies the bridge element of the present invention.

In accordance with the present invention, a source of electrical energy is provided which is capable of applying a high voltage, usually of the order of 1000 to 2000 volts and which has a rate of voltage rise and which stores sufficient energy to explode a conventional EBW, such conditions being referred to for convenience as functioning conditions. A bridge is used in place of the EBW which is dielectric in character, or of low conductance. For convenience this will be referred to as an exploding dielectric bridge. This dielectric bridge is one which will not conduct appreciably at low voltages of the order of a few hundred volts; but at voltages of the order of 2000 volts applied with a fast rise time of the order of a microsecond it will undergo a change a state and, like an exploding bridgewire it will release a large amount of thermal and kinetic energy.

Referring to FIGURE 1 of the drawings, a simple circuit is shown which is greatly simplified compared to actual EBW circuits (many of which are available and are well known). Such circuit comprises a power source 10, a loading resistor 11, a storage condenser 12, a switch 13, bridge terminals 14 and a bridge 15. A similar circuit is shown in Chace and Moore, Exploding Wires, at page l5. The parameters of this circuit are such that, when the switch 13 is closed, a voltage peak of, say, 2000 volts is applied across the terminals 14 in a time of the order of one microsecond. If the wire is of sufliciently small diameter and mass, and if the energy stored by'the condenser 12 is sufficient (e.g., if a platinum wire 0.060 inch in length and having a cross sectional area of 3 X10 sq. inch is used, and if the source is a 1 at. source) the wire 15 will explode. As stated, however, wire 15, being a conductor, will conduct and heat if a lower voltage (e.g., 250 volts AC), is applied, or if a high voltage is applied with a slow rise time, and such heating may initiate an explosive in contact with the wire, or it may melt the wire without initiating the explosive but, in such case, it will render the device inoperative; i.e., it will be a dud.

Referring now to FIGURE 2 identical terminals 14 are shown which are connected to a similar power source (not shown) but which are bridged by the dielectric bridge 16 of the invention which is shown on an exaggerated scale for purposes of description. The bridge 16 is composed of a continuous phase 16a of dielectric material within which are dispersed particles 16b of conductor. It has been discovered that a bridge element of this type has certain advantageous properties. The bridge element 16 conducts appreciably only when a certain minimum voltage is applied; it undergoes the phenomenon of a change of state or explosion with attendant release of a large amount of energy only when a high voltage is applied with a fast rise time and from a power source having an adequate amount of available electrical energy; and it can withstand test conditions such as stand-off voltages of the order of volts without being destroyed. The bridge 16, viewed as an exploding bridge element, is a threshold exploding bridge element; it will not function until certain minimum operating conditions (voltage magnitude, rate of voltage rise and available energy) are applied and, unlike conventional EBWs its conductance is so low under normal conditions that it is not destroyed by conditions (lower voltages, shorter rise times, etc.) which will destroy without exploding a conventional EBW.

The dielectric bridge 16 has a very small cross sectional dimension, of the order of 3X10 to 1.5)(10 square inches; otherwise its mass is such that it requires an excessive amount of electrical energy for a change of state to occur. Preferably the bridge has a trapezoidal cross section.

A very excellent dielectric bridge element can be formed, with a close control over its dielectric properties and dimensions, by forming a fluid or plastic mixture of a dielectric (or potentially dielectric) material and a finely divided conductor, depositing this fluid or plastic mixture in a mold, causing hardening of the mixture to a suitably rigid, solid state and in the form of a rod-or wire-like member, and removing this member from the mold. This technique may be carried out with a gang mold in which a large number'of bridge elements are formed simultaneously. i

For example, a siliceous cement such as a product of Sauercise'n CementsCo'mpany of Pittsburgh, Pa., known as' Sauereisen Insa-Lute High Temperature Cement No. 7 may be employed in the form of an aqueous slurry to which powdered aluminum is added. The slurry is then cast into a mold of suitable dimensions and is cured by heating until all freewater is driven off and the cemetitious ingredients have reacted, set and cured to form a solid rod. This rod is then'removed from the mold and is secured to the output terminals of an EBW power source, e.g., by using a similar slurry of cement.

By this means, and with a reasonable amount of trial and error, a rod can be made and attached to the output terminals of an EBW energy source, which will have the following desirable characteristics: It will not conduct to an appreciable extent unless-a functioning voltage (i.e., the voltage at which the device is intended to function), or a voltage of the same general magnitude is applied. Therefore, if substantially lower voltages are applied the device will not function; i.'e., the bridge will not undergo a change of state and will not release enough energy to initiatea charge of explosive. Nor will the bridge melt and fail. Therefore the bridge provides a threshold, or safe and dud-proof conductance element. Moreover, it has been discovered that even a high voltage of the magnitude of a functioning voltage must have a rise time of very short' duration, of the order of a microsecond. Therefore, application of a high voltage with a slow rise time will not cause either functioning or failure of the rod. Moreover, even if the magnitude and rise time of the voltage are correct '(e.g., a voltage of 2000 volts or more and a rise tim'eof a microsecond or less), the bridge will not undergo a change of state 'with release of a large amount of energy unless a certain minimum energy is available in the power circuit. Under these conditions (a high voltage, a very short rise time but insuflicieht energy) the bridge will conduct but it will not explode and it will not be destroyed.

Therefore, a set of three concurrent conditions must be met for the rod to undergo a change of state and to release enough energy to initiate an explosive, and if any one of these conditions is lacking the rod will not undergo a change of state and it will not be destroyed. The three concurrent conditions are: (1) A minimum, threshold voltage; (2) a very short rise time for the voltage to peak, of the order of one microsecond; and (3) a suflicient amount of available energy.

Thebridge elements of the present invention can be made in many different ways. For example, any solid conductor may be used, such as aluminum, copper, silver zirconium; alloys of these with each other or with other metals; carbon and silicon. Even a liquid conductor such as mercury can be used if dispersed in a dielectric solid.

Likewise any solid dielectric material may be used as the continuous phase, provided it is compatible with the conductor and with adjacent materials and is not unstable, corrosive or too volatile, and provided it can be formed into rods of small diameter or painted on a dielectric surface. For example, various cementitious materials such as water glass, plaster of Paris and Portland cement may be used; also organic thermosetting and cold setting arn-ea resins such as phenol-formaldehyde, urea-formaldehyde and epoxy resins; also silicones, also thermoplastic materials which have a sufliciently high melting point, e.g., rubber; and such miscellaneous materials as sawdust, ground cork, ground glass etc., cemented together with a dielectric cement.

The preferred materials of construction are aluminum (as the conductor) and, as the dielectric, an inorganic cementitious material which can be set and cured to a steady physical state in a short time, such that when a rod is formed it does not undergo a gradual change which substantially alters its electrical properties. The aluminum is preferably a very fine (e.g., 15 micron diameter), very pure (having only a very thin oxide coating) grade or" aluminum in atomized form. The preferred dielectric material is the above-mentioned Sauereisen No. 7, which is available as a paste. Other techniques for fabricating the bridge elements of the invention are available. For example, instead of casting the material into the form of rods while in plastic form, a sheet or block of the material may be prepared and then cut and/ or machined into the form of rods; or the plastic material may be extruded into a rod-like shape; or it may be painted bet-ween terminals and cured in situ; or it may be formed from a mixture of dielectric and metallic (or other conductive) powders and formed into rods by heat and/ or pressure.

The proportions of conductor and dielectric may vary widely according to the nature of the conductor (i.e., chemical composition, particle size and particle shape), the nature of the dielectric, the length of the bridge, the applied voltage and other factors. For example, if the terminals are close together, a lower concentration of conductor is acceptable. In any case, the amount of conductor is limited by the requirement that it be present as the disperse phase, and by the requirement that the bridge function as a dielectric at voltages substantially less than the intended operating voltage. The lower limit of conductor is determined by the requirement that the bridge must break down and become a conductor at a substantially lower voltage than the pure dielectric.

The following specific examples will serve further to illustrate the practice and advantages of the invention.

Example 1.l0 grams of Sauereisen Paste No. 7 are mixed with 1.5 grams of Sauereisen Thinner No. 14 (the latter being a 37% water glass solution). (The paste contains about 68% water insoluble solids and about 10% Water soluble solids, the balance being water.) Water is added with stirring until the mixture is slightly thinner than putty. 4.9 grams of powdered aluminum (atomized, Reynolds Grade No. 200) are added. Then the mixture is blended thoroughly and cast in a mold to form rods approximately 0.040" x 0.040" in cross section which are baked while in the mold at 250 F. for 45 minutes. The rods thus formed are removed from the mold and applied to the terminals of anEBW device as follows: The terminals and the ends of a rod are buttered with the same material used to form the rod, and the ends of the rod are caused to adhere to the terminals. The device is then heated at 250 F. for 45 minutes. The bridges thus tormed are cleaned with aqueous acetic acid solution followed by rinsing with distilled water, after which they are heated again at 250 for 2 hours.

A typical bridge of the character described will perform as follows: It will withstand voltages of 110 and 250 volts A.C. When subjected to a test wherein a 20 kv. voltage having a very short rise time (less than one microsecond) from a 500 ,uuf. source is applied, the bridge will are but will not be impaired. In fact, such high voltage, low energy treatment is an aid in curing and stabilizing the bridge. The bridge will stand otf a DC. voltage of 400 volts. The bridge will function (i.e., it will explode, releasing a large amount of energy and initiate a charge of pyrotechnic explosive such as a mixture of potassium perchlorate and 20% pyrotechnic aluminum or a mixtureof potassium nitrate and boron), when subjecte as the conductor. We have made and successfully tested bridges made from other dielectrics and other conductors.

For example bridges have been prepared of 57.6% aluminu-m powder and 42.4% "epoxy resin. Eight out of ten such bridges showed infinite resistance. Two had a resistance of about 600 ohms, but oncuring one of these .at 250 F. for one-half hour it developed an infinite reduct, not that it was destroyed. .The breakdown voltages of the above epoxy and Zr-Ni bridges was considerably lower than the breakdown voltages of Sauereisen-Al bridges (about 4500 to 5000 volts D.C.,"steady state). Therefore the Sauereisen-Al bridges were preferred, but for many purposes the bridges described above (i.e., epoxy-Al and S-auereisen cement-Zr-Ni bridges) are satisfactory. 7

Referring now to FIGURE 3, a commercial igniter is there shown which embodies the present inventi'onf An initiating device is shown which is generally designated by the reference numeral 20. It comprises a metal body 21 having threaded portions 22 and 23 which are intended to be screwed into associated equipment. For example, the portion 22 may be screwed into an EBW power source and the portion 23 into a propellant device which is to be initiated. Pins 25 are provided Whose right-hand ends are connected to the EBW power source (not shown) and whose left-hand ends form bridge terminals. The pins 25 are sealed in a glass plug 26. A static discharge ring 27 and a resilient O-ring 28 are also provided.

A'dielectric bridge 30 is provided in accordance with the present invention which is connected to the inner (term-inal) ends ofthe pins 25; The bridge 30 is in contact with a pyrotechnic material 29 such as a mixture I of potassium perchlorate and pyrotechnic aluminum. A

ceramic bushing is provided at 35 and a mica disc at 36 which separates the pyrotechnic material 29 from a body of a base charge of igniter material 37 such as mixtures of the molybdenum oxides and aluminum powder in an aluminum cup 38.v When the EBW power source .is operated under the proper conditions, e.g., 2000 volts with a rise time of 1 microsecond from :an 0.5 ,wf. source, the bridge 30 will undergo 'a change of state with a large release of exploding energy sufiicient .to initiate the pyrotechnic material 29 which in turn will initiate the mixture 37. The latter will, in turn, ignite a rocket motor or perform some other desired function. The device will not operate, nor will it be made inoperative, by stray or accidental currents which would fuse the wire of a conventional EBW device.

It will, therefore, be apparent that an EBW device, and

a bridge element therefor, have been provided which have certain desirable threshold characteristics which prevent,

or minimize the likelihood of unintended operation and of fusing and [failure of the bridge element, but which operate reliably when the operating conditions are applied.

We claim:

1. An exploding bridge device comprising,

a nonconductive base member;

a of mutually insulated conductive terminals extending through said base member;

.a normally-noncond-ucting dielectric bridge connecting said terminals andsupported upon said base memher, said bridge element including an elongated rodlike portion consisting essentially only of a hardened water glass solution having dispersed therethrough a plurality of metallic aluminum particles, each of said 'iparticles exhibiting an average diameter of about 15 microns and being insulated from adjacent ones of said particles by said hardened water glass solution, said bridgeelement having a cross-sectional dimension within the range 3 x 10 to 1.5 x 10 square inches, said dispersedaluminum particles being present in suificient quantity that said bridge element explodes only in response to a voltage pulse in excess of 2000 volts with a rise time of the order of one micro-second applied from a 0.5 ,uf. source coupled to said pair of terminals. 7

2. The device of claim 1 wherein said bridge is embedded in a pyrotechnic material.

References-Cited by the Examiner UNITED STATES PATENTS SAMUEL FEINBERG, Primary Examiner.

A. M. HORTON, Examiner. 

1. AN EXPLODING BRIDGE DEVICE COMPRISING, A NONCONDUCTIVE BASE MEMBER; A PAIR OF MUTUALLY INSULATED CONDUCTIVE TERMINALS EXTENDING THROUGH SAID BASE MEMBER; A NORMALLY NONCONDUCTING DIELECTRIC BRIDGE CONNECTING SAID TERMINALS AND SUPPORTED UPON SAID BASE MEMBER, SAID BRIDGE ELEMENT INCLUDING AN ELONGATED RODLIKE PORTION CONSISTING ESSENTIALLY ONLY OF A HARDENED WATER GLASS SOLUTION HAVING DISPERSED THERETHROUGH A PLURALITY OF METALLIC ALUMINUM PARTICLES, EACH OF SAID PARTICLES EXHIBITING AN AVERAGE DIAMETER OF ABOUT 15 MICRONS AND BEING INSULATED FROM ADJACENT ONES OF SAID PARTICLES BY SAID HARDENED WATER GLASS SOLUTION, SAID BRIDGE ELEMENT HAVING A CROSS-SECTIONAL DIMENSION WITHIN THE RANGE 3 X 10**-6 TO 1.5 X 10**-3 SQUARE INCHES, SAID DISPERSED ALUMINUM PARTICLES BEING PRESENT IN SUFFICIENT QUANTITY THAT SAID BRIDGE ELEMENT EXPLODES ONLY IN RESPONSE TO A VOLTAGE PULSE IN EXCESS OF 2000 VOLTS A RISE TIME OF THE ORDER OF ONE MICRO-SECOND APPLIED FROM A 0.5 UF. SOURCE COUPLED TO SAID PAIR OF TERMINALS. 